Inhibitory effect of lipopolysaccharide on immune response after DNA immunization is route dependent

The DNA prepared from E. coli contained high levels of lipopolysaccharide (LPS). When antigen-encoding DNA was injected into mice, toxicity and increased IgM responses were observed, A method for purifying high yields of DNA (up to 12 mg/L of broth culture) with very low levels of LPS (0.05 ng/mg) was developed. When this purified DNA was used for immunization studies, the toxicity and increased IgM responses were abrogated. Thus, LPS was added to DNA in order to examine its influence on the IgG and cytotoxic T lymphocyte (CTL) response after intramuscular (i.m.) or intradermal (i.d.) DNA immunization. The IgG response to DNA-encoded antigen was inhibited in a dose-dependent manner by the i.d., but not the i.m., route of immunization. Surprisingly, no effect on the CTL response was observed. Therefore, the ability to produce high yields of plasmid DNA with very low levels of endotoxin contamination is advantageous for DNA immunization studies, not only for toxicologic but also for immunologic considerations. Furthermore, these results provide further evidence that immune induction occurs via different mechanisms after i.m. and i.d. DNA immunization.